Wireless digital communications systems are poised to offer a cost-effective alternative to cable and Digital Subscriber Line (DSL) technologies or data services. One example of wireless digital communications systems is the Worldwide Interoperability for Microwave Access technology, or so called “WiMAX” technology. The WiMAX technology is based on the IEEE 802.16e air interface standard and is a promising framework for broadband wireless applications. WiMAX has the potential to enable full internet and digital voice services for both fixed and mobile users.
The WiMAX network architecture includes a subscriber station (SS) that communicates with a base station (BTS) via a wireless link or interface. The BTS includes PHY and media access controller (MAC) functionality, where PHY functionality takes care of encoding and decoding between a pure digital domain and a modulation in the analog domain. WiMAX uses Orthogonal Frequency Division Multiplexing (OFDM) because of the robustness to multipath propagation offered by OFDM and the use of multiple antenna systems. Orthogonal frequency-division multiplexing (OFDM) is a modulation technique for transmission based upon the idea of frequency-division multiplexing (FDM) in which each frequency channel is modulated using a simpler modulation. In OFDM the frequencies and modulation of FDM are arranged to be orthogonal with each other which reduces or eliminates the interference between channels.
OFDM is based on the theory that because low-rate modulations (modulations with relatively long symbols compared to the channel time characteristics) are less sensitive to multipath, it is better to send multiple low rate streams in parallel rather than sending one high rate waveform. This is what OFDM is doing because in operation it divides the frequency spectrum in subbands small enough so that the channel effects are constant (flat) over a given subband. A conventional modulation scheme (e.g. BPSK, QPSK, M-QAM, etc.) is then used to send information over the subband, and the fast changing effects of the channel (multipath) are significantly reduced or eliminated as they are now occurring during the transmission of a single symbol and are thus treated as flat fading at the receiver.
WiMAX realizes OFDM along with use of multiple antennas. The multiple antenna systems used in WiMAX include the use of multiple transmit and multiple receive antennas to provide a multiple-input multiple-output (MIMO) system. The MIMO system is a multi-antenna communication system that makes significant increases in throughput and range possible at the same bandwidth and same overall transmit power expenditure. In general, MIMO technology increases the spectral efficiency of a wireless communication system. Wireless MIMO communication exploits phenomena such as multipath propagation to increase data throughput and range, or reduce bit error rates, rather than attempting to eliminate effects of multipath propagation as traditional SISO (Single-Input Single-Output) communication systems seek to do. MIMO multiplies the point-to-point spectral efficiency by using multiple antennas and radio frequency (RF) chains at the BTS and the SS. MIMO achieves a multiplicative increase in throughput relative to SISO systems by carefully coding the transmitted signal across antennas, OFDM symbols, and frequency tones; this increase in throughput is generally realized without impact on system bandwidth or transmit power.
It has thus been shown that the channel capacity (a theoretical upper bound on system throughput) for a MIMO system is increased as the number of antennas is increased, proportional to the minimum number of transmit and receive antennas. The transmit side of typical MIMO systems uses multiple transmit antennas, while receivers can include single or multiple antennas. In a multiple antenna transmit system, the transmitter typically sends different signals from each transmit antenna, and these signals are referred to as streams.
In a conventional MIMO system the number of transmit signals or streams is equal to the number of antennas, however in some cases the number of transmit antennas used is greater than the number of transmit streams. In such systems there is consequently a need to make “M” real antennas appear as “N” virtual antennas (N<M) with each virtual antenna driven by one transmit stream. The virtual antenna must effectively behave like a real antenna and should reasonably cover the angular pattern of the real antenna. The advantages of virtualization include but are not limited to improved diversity in the link and lower power amplifier rating (but with larger number of antennas).